Electromagnetic pickup



May 10, 1949. H. F. SCHWARZ ELECTROMAGNETIC PICKUP Filed'Oct. 19, 1945milk . INVENTOR. wen/5y Fc/WAEZ 14f or/26y Patented May 10, 1949 UNITEDSTATE$ OFFICE 14 Claims.

the vibrations recorded on the sound track. It i must have extremerigidity in the axial direction or lengthwise of the sound track tominimise distortion. In pick-ups of the electromagnetic type it is alsorequired that the magnetic flux vary linearly with the from anintermediate or neutral position. Furthermore, to prevent resonanceeffects and similar forms of distortion, it is required that the naturalperiod of lateral vibration of the stylus be well below the lowestfrequency desired to be reproduced and it is also necessary that thepick-up assembly and stylus each have a mini mum weight and inertia tominimise the loading on the walls of the record grooves, particularly athigh frequencies or sounds having steep wave fronts, and in the eventthe record is not quite flat. As a matter of convenience it is alsopreferable to arrange for the ready replacement of the stylus.

The reproducers of the character referred to known prior to my inventiondo not meet these desirable conditions. In most constructions the stylussupport depends upon the use of trunnions or upon the relative stiffnessin two directions of a thin, fiat member to provide rigidity in thelongitudinal direction together with lateral flexibility. such aconstruction provides at best a compromise in which insufiicientlongitudinal rigidity is obtained and in which the stylus has greaterrigidity in the lateral direction than is desirable. This leads todistortion resulting from the longitudinal displacement of the stylus,and from the resonance of the stylus when reproducing low frequencies. Alinear relation between the rate of change of flux and the displacementof the stylus is seldom obtained because of the prior attempts to usehighly efficient magnetic circuits. Also in the prior constructionsprovision was made for replacing the stylus by a thumb screw type ofclamp which added greatly to the weight of the stylus assembly andresulted in excessive loading on the sound track walls and rapid wearingof the records.

It is accordingly an object of my invention to provide a reproducerorpick-up of the electrodisplacement of the stylus Great Britain June 18,1945 magnetic type which overcomes the above noteddisadvantages byreason of a novel magnetic circuit and an improved stylus mounting.

It is another object of my invention to provide a pick-up of thecharacter set forth in the preceding paragraphs in which a materialreduction in weight is obtained by the use of a greatly simplifiedmagnetic circuit.

It is also an object of my invention to provide a pick-up of thecharacter referred to in the preceding paragraphs-in which the stylusholder is made extremely light in'weight and constructed to have highintrinsic rigidity.

It is an additional object of my invention to provide a pick-up of thecharacter referred to in the preceding paragraphs in which the stylusassembly is formed with a T-shape, lateral vibration of the stylus beingaccomplished by pivotal movement of the assembly about the longitudinalaxis of the bar of the T, such movement being permitted by trunnionblocks of rubber or similar resilient material adhesively bonded to theextremes of the bar of the T-shape, whereby the relatively great lengthof the bar of the T effectively prevents movement of the stylus in alongitudinal direction.

It is also an object of my invention to provide a pick-up of thecharacter heretobefore described which includes a pair of simple opposedpermanent bar magnets for establishing the required magnetic field.

It is a still further object of my invention to provide a pick-up of thecharacter referred to hereinbefore in which the permanent magnet polepieces are disposed between the rock bar and the record engaging end ofsaid stylus, and as close to said stylus as possible,

It is an additional object of my invention to provide a pick-up of thecharacter set forth in the preceding paragraphs which is formed as apair of separable hinged halves normally held in closed position by adetent mechanism and which includes means operative in the closedposition to hold the stylus in the proper relation to the other parts ofthe pick-up through clamping pressure exerted on the rubber trunnionblocks by the two halves of the assembly.

Other objects and advantages of my invention will be apparent from astudy of the following specification read in connection with theaccompanying drawings, wherein;

Fig. 1 is a perspective drawing illustrating the general form andappearance of a pick-up constructed in accordance with the preferredembodiment of my invention;

Fig. 2 is a longitudinal sectional view through the pick-up shown inFig. 1 and illustrating the details of construction of a coil supportand stylus mounting;

Fig. 3 is an underside view of the device shown in Fig. 2 and furtherillustrating the relative positions of the parts;

Fig. 4 is a fragmentary cross sectional view taken along the line IVIVof Fig. 2; and

Fig. 5 is a perspective view of the stylus assembly showing the detailsof construction.

Referring to the drawings, I have illustrated therein the preferredembodiment of my invention as comprising a pick-up assembly formed astwo separable halves I and 2 joined to each other by a hingedconstruction comprising a tongue 3 on the lower member 2 disposed withina longitudinally extending slot 4 formed in the upper member I, a hingepin 5 being passed longitudinally through the tongue 3 and the bosses 6and 1 disposed on either side of the tongue 3 and defined by the slot 4.The upper part I of the assembly constitutes a cover for the lower partand serves also as a clamping mechanism for holding the stylus in anoperative position as will subsequently appear.

When in use, the hinged halves are moved into juxtaposition asillustrated in Fig. 2 and are held in such position by a detentmechanism which may comprise a projection 8 formed on the upper half Iand which enters a similarly contoured depression formed in the lowerhalf 2.

The rear portion 9 of the upper half I is givena cylindrical form and isbored as indicated at II] and I to provide for attachment of a tone armor swivel mounting providing for the traversal of the pick-up over thesurface of the record during reproduction.

The bore I communicates with an interior space I2 permitting theelectrical conductors comprising the output circuit of the pick-up to beconnected respectively to a pair of connector elements l3 extending in agenerally vertical direction through the lower half 2 and extending intoa recess I4 formed in the under surface of the lower half. This recessand an upwardly extending bore serve to house a pick-up coil l6 and maybe normally closed by a dust cover l1 removably secured .to theunderside of the lower half 2 as by means of fiat head countersunkscrews IS.

The bore I5 preferably extends somewhat beyond the mid line of the lowerhalf 2 at which point its diameter is reduced as indicated at I9 toprovide a shoulder against which the pickup coil l6 may be seated. Thepick-up coil I6 may be secured within the recess I 5 as by means ofshellac, varnish or othe suitable adhesive.

As appears in Figs. 2 and 4, the reduced diameter bore I9 communicateswith a longitudinally extending recess 2| which is preferably given arectangular cross section and which serves to house a stylus assemblyindicated generally by the reference character 22. The stylus assemblyis preferably constructed along the lines illustrated in Figs. 2, 4 and5 and comprises a formed T shaped member consisting of a rock barportion 23 from the center of which depends a tapered cylindrical tube24.. The tube 24 is preferably contoured as shown in Fig. 4, to providea short, straight cylindrical section 25 merging into a conical section26 which is secured to or formed integrally with the transverse barportion 23. The depending tubular portion 24 is preferably bored asindicated at 21 to receive a 4 sound track engaging stylus 28' formedpreferably of sapphire Or a similar jewel.

The bar portion 23 is by preference given a channel shaped cross sectiondefined by a horizontally extending web 29 from opposite sides of whichrise narrow flanges 30 and 3|. The flanges 30 and 3| at the extreme endsof the bar portion 23 are given a somewhat greater width and are turnedinwardly toward each other as shown at 32 and 33 to provide aquasicylindrical section .to which trunnion blocks 34 and 35 formed ofrubber or other like resilient material are secured, preferably as bymeans of a vulcanising or adhesive bonding process.

I have found that the complete stylus holder can conveniently be formedby a drawing operation-as for example by a machine similar to an eyeletforming machine.

The stylus assembly is mounted within the lower half 2 in the mannershown in Figs. 2 and 4 with the trunnion blocks 34 and 35 seated withinthe channel 2|. The upper half I includes downwardly extending bosses 36and 31 disposed to engage the upper surfaces of the blocks 34 and 35respectively when the halves are in their closed positions to therebyexert a clamping pressure on the blocks to hold the stylus assemblyfirmly within the recess 2 I. With the parts so positioned the tubularportion of the stylus passes through the center of the coil I6 andextends below the lower cover I! a suflicient distance to permit thestylus to engage the record surface and allow: adequate clearancebetween the cover and the surface of the record.

By properly locating the channel 2| and by carefully controlling thelocation and alignment of the blocks 34 and 35 as through use of a jigor fixture during bonding or vulcanizing, proper alignment upon assemblyis assured.

The ends of the coil I6 are preferably carried through the recess I4 andsecured as by soldering to the exposed ends of the connector member I3,thus completing the electrical circuit between the coil and the outputconductors.

The magnetic field for the device is furnished by a pair of simplerectangular permanent bar magnets 38 and 39 secured in any suitablemanner within rectangular recesses 40 and 4| formed in the under surfaceof the lower half 2 and aligned with the coil supporting bore I 5. Thedepth of th recess I4 is chosen to place the upper surface of themagnets 38 and 39 substantially in contact with the lower surface of thecoil I6. While the magnets 38 and 39 can be secured within the recesses40 and 4| by any suitable means, as 'by means of screws or clamp-s, Ihave found that an adequately secure mounting is provided by making therecess and magnet dimensions conform closely and by using a suitableadhesive for holding the magnets in place.

The magnets are placed with unlike poles on opposite sides of the stylusassembly 22 and are spaced apart a distance substantially equal to theinternal diameter of the coil IS, a distance which is somewhat greaterthan the maximum amplitude of vibration required of the stylus 22.Furthermore the magnets are given a width in the longitudinal direction(at right angles to thedirection of stylus motion) which is largecompared to the diameter of the stylus. By this means there is producedacross the gap between" the opposed faces of the bar magnets asubstantially uniform magnetic field of considerable strength. Theexternal magnetic circuit comprises the leakage path through the air andthroughthe-hody oithepick-up fromthe extreme ends of the magnet...Thestuhular: portion; 2! oi. the stylus-assembly constit= tesia shunt:magnetic circuit; joiningrthe midpoin-ts of: the air gap -andexternal'portions of the magnetic! circuit. There. fore, when the stylusportion: N positioned centrally between adjacent laces oi the magnets3.8. and; 38 substantiallyno time will flow through the stylus However,a; lateral displacement oi the stylus to bringthe stylus nearer oneofz.the magnets: and: further irom the: other will urn balance the magneticfield. and cause: a to flow in one direction the stylus A displacementthe opposite direction will result in: a. flux flow through the-stylusin the reverse direction. Because the magnetic. field extending frompole to: pole at the: magnets. 38; and: 39' uniform and because theextent of the: lateral movement of. the is nonnally smaller than: thebetween the adjacent magnet faces the: flux changethroughthci stylus:hearse substantially' linear relationship to the magnitude oi. thedisplacement:

I have found that with 'a. pick-up-constructed as; described, the stylushas: great flexibility in the laterai direction and a period ofviluration in that: direction which-is substantially lower than the.lowest: recordnfire ueuey desired to be reproduced. Furthermore, thestylus. pre sents extreme: rigidity inthe longitudinal direc ti-cmandis-not subdected to any material dis.- placement in the iongitudinaldirection. during: operation. Faithfiul reproduction may be ohtained upto: iremrenciesas as 12,060 cycles per second.

Attention is. iurthenno-re directed to the ease. with which the stylusmay be replaced as may he required by thewear resulting from continueduse of the device. Replacing a: stylus is accom.-- plishedsimplyaxndi-eaisily hyhingedly 'movingithe halves l' and 2 awayfromeach. other whereby the complete stylus assembly 27." may lie-liftedupwardly from the channel 21 and replaced by a new stylus of likeconstruction- Als'c:r,..t;lr1ev total weight of the pick-up is comsiderazbly reducedv by the simplified magnet struc:-- turedisclosedherein;

While Ihaveshown and'descrihed the. preferred embodiment of myinvention 1. do not desireto: be limited to the details have been: shownand describedherein except as defined in the appended claims.

I claim l. In an electromagnetic phonograph pick-up,

the combinationofi: abody member formed of a light weight non-magneticmaterial; a pick-up coil mounted in a recess formed. in one surface ofsaid body; an armature of magnetic material extended axially throughsaid coil; and a pair of simple permanent bar magnets disposedonopposite sides of said armature. and extending along a. line normal tothe length or said armature, said magnets being secured within an.elongated. channel formed in said one surface of said body memher, saidmagnets extending over the surface of said coil to hold said coil withinsaid recess.

2. In a stylus assembly for an electromagnetic phonograph pick-up, thecombination of: :a, tubular stylus shank formed 'of magnetic material; apointed record engaging stylus secured in one end of said shank; arock-bar secured to the other end of said shank at right angles theretoand extending substantially equal distances from said shank; and a pairof trunnion blocks secured to the ends of said rock bar, said trunnionof construction which blocks being. termed of 1 a. resilient materiai.ham a. low: modulus" or rigidity and great elasticity permitting angularmovement, or Sliiifdizlflfilfi bar in response to lateraldis Taoism-cut:said stylus.

' 3. In a. stylus; assembly for an. electromagneticph-cnograsphpiclc-up, the: cembmation of: arm bular stylus shame termedof magnetic: material;

a. pointed record engaging'stylus secured. in one end oi said shank; arock-bar secured to other end. of said shank. at right angles theretoand: extending '"substantially equal distancesafrom said shank; andapairi of trunnion blocks -ad hesively' honded' to. the ends oi saidrock; bar, said trunniomblocks being formed: of a. resilient? mm terialvhaving alovw mod; elasticity: permitting angular movement. 01? said rockbar in response to lateral displacement oi said-stylus d; In-a;.-styl:us essemloly ioran electromagnetic phonographzpick-up, thecombination of: a tuslisularstylus shank: formed or magnetic material: apointed record engaging stylussecured: in one end or said shank; a. rockbar secured t c-the other end-ofsaidshank' at right anglesthcreto andextending substantially equal distances: from said shank, said rochbar-having a-channel shaped section disposed withthe plane of the. webthereof at rightangl'es to said shank; aspair o1 trunnion blocks securedto the en de of said rock bar, said trunnion: blocks being formed of" a.resilient-material -having a low-modulus of rigidity and? greatelasticitypermitting angular movement ofsaid rock bar in response tolaterali dli'splacement of said stylus.

5; In a; stylus assemblyfor anelectromagnetic phonograph pick-up, thecombination of: a: rock her of channel shaped cross section; trunnionblocks formed of "a resilientmateriali having physteal propertiessimilar to those of vulcanized rubberadhesively bonded to oppositeends-ci-said' rock bar: and a; tubular stylus shank secured tosaid rockbar intermediate the ends thereof-and at right angles te -the lengththereof, saidshanlc having a generally tapered form disposed withthelarger' end at said rock bar, said shank haw"- ing asubstantiallyconstant wall thickness alon its length;

6. In a stylus assembly for an electromagnetic phonograph pick-up, thecombination of": a rock bar of channel shaped cross section; trunnionblocks formed or a resilient material having physs lead propertiessimilar to those of vulcanized rubber adhesively bonded to opposite endsof said rockbar: and a tubularstyius shank secured tosaidrcch barintermediate the ends th-ereoi and at right angles to the lengththereof; said shank having a cylindrical section spaced fromsaid-rochbarhy a-frusto-conical section disposed with the largerbase atsaid rack bar,- said shank having a substantially constant wallthickness along-itslength.

7; In a styius assemblyfor an electromagnetic phonograph pickup, thecombination of: atu hu-iar stylus shank: formed of magnetic material: apointed record engaging stylussecu-red in one end of said shank; a, rockbar secured to the other end of said shank at right angles thereto andextending substantially equal distances from said shank, said rock barhaving a channel shaped cross section disposed with the plane of the webthereof at right angles to said shank, the ends of said rock bar havinga quasi-tubular form defined by upwardly and inwardly curving legextensions. of said channel shape; and a pair of trunnion blocks securedto the ends of said rock his of rigidity andgreat bar, said trunnionblocks being formedof a re- 'silient material having a low modulus ofrigidity and great elasticity.

8. In an electromagnetic phonograph pick-up, the combination of: a bodymember formed of a light weight non-magnetic material; a pick-up coilmounted in a pocket extending inwardly from one surface of said body; astylus assembly comprising a rock bar, a shank portion extended at rightangles to said rock bar and disposed intermediate the ends thereof, andelastic trunnion blocks'bonded to the ends of said rock bar; recessmeans in the other surface of said body for receiving said blocks andholding said stylus assembly in a position extending said shankaxially'through said coil: and means clamping said trunnion blocks insaid recess.

9. In an electromagnetic phonograph pick-up, the combination of: a bodymember formed of a light weight non-magnetic material; a pick-up H coilmounted in a pocket extending inwardl from one surface of said body; astylus assembly comprising a rock bar, a shank portion extended at rightangles to said rock bar and disposed midway of the ends thereof, andelastic trunnion blocks bonded to the ends of said rock bar; and meansfor clamping said blocks against the other surface of said body inpositions extending said shank axially through said coil and disposingthe length of said rock bar at right angles to the direction of desiredmovement of said stylus shank, whereby said desired movement ispermitted by the angular movement of said rock bar torsionallydistorting said elastic blocks.

10. In an electromagnetic phonograph pick-up,

the combination of: a body member for-med of a light weight non-magneticmaterial; a cover member hingedly attached to said body member forswinging movement between open and closed positions; a pick-up coilmounted in a pocket extending inwardly from One surface of said body; astylus assembly comprising a rock bar, a shank portion'extended at rightangles to said rock bar and disposed midway of the ends thereof, andelastic trunnion blocks bonded to the ends of said rock bar; recessmeans in the other surface of said body for receiving said blocks andholding said stylus assembly in a position extending said shank axiallythrough said coil; means on said cover member disposed to engage saidblocks in said closed position of said cover and clamp said trunnionblocks in said recess; and means for releasably holding said cover insaid closed position, whereby moving said cover to said open positionpermits removal and replacement of said stylus assembly.

11. In an electromagnetic phonograph Dick-up, the combination of: a bodymember, a stylus assembly comprising a tubular stylus shank formed ofmagnetic material, a pointed record-engaging stylus secured in one endof said shank, and a rock bar secured to the other end of said shank atright angles thereto and extending substantially equal distances fromsaid shank; and resilient mounting means interposed between each of theends of said rock bar and said body flexibly se curing said stylusassembly to said body in a position disposing the length of said rockbar at right angles to the direction of desired movement of said stylusto thereby permit angular movement of said rock bar in response tolateral dis placement of said stylus.

12. In an electromagnetic phonograph pickup, the combination of a bodymember, a stylus assembly comprising a tubular stylus shank formed ofmagnetic material, a pointed record-'- engag-ing stylus secured in oneend of said shank, and a rock bar of channel shaped cross sectiondisposed with the plane of the web thereof at right angles to said shankand extending substan ti-ally equal distances from said shank; andresilient mounting means interposed between each of the ends of saidrock bar and said body flex ibly securing said stylus assembly to saidbody in a position disposing the length of said rock bar at right anglesto the direction of desired movement of said stylus to thereby permitangular move-' ment of said rock bar in response to lateral displacementof said stylus.

13. In an electromagnetic phonograph pick-up, the combination of: .abody member; a stylus assembly comprising a rock bar of channel shapedcross section, and a tubular stylus shank secured to said rock barintermediate ends thereof at right angles to the length of said shankhaving a generally tapered form disposed with the larger end at saidrock bar, said shank having a substantially constant wall thicknessalong its length; and resilient mounting means interposed between eachof the ends of said rock bar and said body for flexibly securing saidstylus assembly to said body.

14. In an electromagnetic phonograph pick-up including a body membercarrying means defining a pair of spaced magnetic pole facesjthecombination of? a stylus assembly including an armature portion and-arock bar portion extend ing transversely of said armature portion;resilient blocks of polygonal cross section bonded to the ends of saidrock bar portion; and means on said body member for engaging said blocksand normally holding said assembly in a position disposing said armatureportion centrally between said pole faces and disposing said rock bar ina position disposing the length thereof at right angles to the directionof desired movement of said armature portion.

HARVEY F. .SCHWARZ.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

